In the manufacturing of thick film resistors, the firing temperature and firing conditions are determined by the conductor material employed, for example, noble metal or base metal materials, and their melting points; moreover, various resistors are used for the desired resistance ranges and the firing condition is constrained also by their resistor compositions. Recently, various improvements are made on these thick film resistors and their manufacturing methods.
For instance, as described in Japanese Kokai Patent Hei 3 (1991)-52202, the following method is known: After the resistor paste, such as RuO.sub.2 series and Pb.sub.2 Ru.sub.2 O.sub.6 series, is printed and dried on the substrate, it is fired at 700-1000.degree. C. in the oxidizing atmosphere; and after the conductor paste comprising Ag alone or a mixture of Ag with one or more selected from a group consisting of Au, Pd and Pt being dispersed in the vehicle, such as Ag-Pd series and Ag-Pt series for low temperature firing, which contains Ag as the main component is coated (printed) and dried in a predetermined position with respect to the resistor layer formed on the substrate to partially overlap the fired resistor layer, it is fired at temperature in the range of 500-700.degree. C. in the same oxidizing atmosphere to manufacture a chip resistor. In particular, it suppresses diffusion of the Ag component toward the resistor film by lowering the firing temperature of the Ag conductor paste from the resistor paste firing temperature.
Japanese Kokoku Patent Hei 5 (1993)-53284 describes a manufacturing method in which, for example, a resistor paste comprising RuO.sub.2 is screen-printed and fired in the oxidizing atmosphere to form a resistor film; a paste; which contains as the conducting component a base metal that can be fired in the temperature range lower than the firing temperature of the resistor film, for instance at 500-600.degree. C., is used to print so as to partially overlap the resistor film edge, and fired in the nitrogen atmosphere to form the conductor pads (terminations). In order to form the base metal conductor, firing must be done in the reducing or inert atmosphere. The conductor oxidation and degradation are prevented by firing at the temperature lower than the firing temperature of the resistor film. The conductor paste comprising the base metal such as Cu must be fired in the reducing atmosphere; the resistor paste fired in the same reducing atmosphere is not only expensive but the temperature coefficient resistance (TCR) of the resistor obtained is poor and the resistance value range is extremely narrow. Therefore, the fact that both the conductor paste and resistor paste can be fired in the air not only simplifies the manufacturing method but also reduces the resistance value change over a wide range of resistance values and can give resistors which are excellent in resistance characteristics and also economically advantageous. Hence, the conductor paste with Ag as the main component can be used together with a resistor paste that can be fired in air.
In the manufacturing method of this conventional thick film resistor and the thick film resistor structure in which part of the conductor forms an overlapping joint with a resistor formed on the substrate on its both edges, it has never been sustained that performances originally required for such conductor pads, for instance, high strength of adhesion to the insulation substrate and sulfurization resistance are provided and uniform and accurate resistance characteristics over a wide resistance range, small resistance value changes caused by the environmental change or manufacturing process and reliability of the resistor are maintained; since there is apprehension on these points no commercialization has been attained.
Thus, it is desired that when the uniform and same resistance material is used, a thick film resistor is formed with desirable resistance characteristics of high accuracy even in different sizes and small resistance value changes as reliability and is provided with conductor pads of well-balanced high adhesion strength and sulfurization resistance, and the manufacturing method thereof is provided. Consequently, the objective of the present invention is that, in the thick film resistor and the manufacturing method thereof, in which the Ag series thick film conductor paste as the conductor material and the resistor paste which can be fired in the oxidizing atmosphere are used, and each of them is printed and fired on the insulation substrate so as to form a resistor (resistor film) between the conductors, the above-mentioned problems of the current technology are solved, and a thick film resistor of high performance and high reliability, and its manufacturing method are presented.
In view of the above-mentioned situation, as a result of zealous investigation in order to solve the above-mentioned problems the present inventors found the following: A thick film resistor and the manufacturing method thereof in which (1) a resistor layer is formed on surface of a insulation substrate, (2) a pair of conductor pads of Ag conductor composition comprising palladium and/or platinum is disposed with predetermined spaces from said resistor layer to sandwich said resistor layer in a direction of its conductive resistance path on the surface of insulation substrate, and (3) Ag conductor layer of Ag conductor composition without palladium and platinum is disposed over both said resistor layer and conductor pads at their respective edges to connect electrically said resistor layer to said conductor pads to form the conductive resistance path gave small resistance value changes, a small TCR in any sizes of resistors and improved the resistance value yield and, furthermore, could give high adhesion strength between the insulation substrate and conductor pads, and high sulfurization resistance to solve the above-mentioned problems. Thus the present invention was achieved.